The mechanism of transduction in invertebrate (Limulus) and vertebrate photoreceptors will be studied. Four separate projects are being attempted. 1. The development and application of a method for internally dialyzing photoreceptors. This method allows more rigorous and reversible control of the cytoplasmic composition than previous methods. We will use this method to test the involvement of cyclic nucleotides in transduction in Limulus. We will attempt to apply this method to vertebrate rods. 2. Using a suction electrode we can record light-induced currents through small patches of the photoreceptor membrane. We are attempting to reduce the noise of these measurements so that single-channel currents can be correlated. 3. An attempt will be made to isolate an active factor from extracts of illuminated squid retinas which open light-activated channels when introduced into Limulus photoreceptors in the dark. 4. The shape of quantum bumps is being studied in Limulus with the goal of developing a kinetic theory of transduction.